1. Field of the Invention
The present invention relates to an imaging lens system and, particularly, to an imaging system of two-lens structure, which is capable of reducing the size and weight and improving the optical performance and productivity. The imaging system is used for an image-taking device that forms images of objects such as scenery and human figures on an image taking surface of a solid image sensor element such as a CCD, CMOS, etc., which is mounted on a portable computer, a television telephone, a cellular phone, and the like.
2. Description of the Related Art
Recently, there has been an increasing demand for reducing the size and weight and improving the optical performance (high resolution of the solid image sensor element, etc) of an image-taking device (camera) that utilizes a solid image sensor element such as a CCD, CMOS, or the like, which is mounted on a portable telephone, a portable computer, and a television telephone, for example.
Therefore, it is also necessary for the imaging lens system used for such image-taking device to be small and light. In addition, the imaging lens system is required to have an excellent optical performance in order for the solid image sensor element to exhibit the sufficient resolution capacity.
In order to meet such demands, recently, there has been a spread of a lens system with two-lens structure that is small, light, and capable of exhibiting a more excellent optical performance than that of a single-lens structure lens system. It is expected that the importance of such lens system with two-lens structure will increase more and more in the future.
As the lens system with two-lens structure, there have been known the lens systems disclosed in Patent Literatures 1–4.
[Patent Literature 1] Japanese Patent Unexamined Publication 2004-4742
[Patent Literature 2] Japanese Patent Unexamined Publication 2003-329921
[Patent Literature 3] Japanese Patent Unexamined Publication 2003-227999
[Patent Literature 4] Japanese Patent Unexamined Publication 2003-75719
Each fundamental structure of the lens systems disclosed in Patent Literatures 1–4 belongs to one of the three types 1–3 described hereinafter.
(Type 1)
A lens system in which an object side lens (referred to as a first lens hereinafter) is a biconvex lens with a positive power, an image surface side lens (referred to as a second lens hereinafter) is a meniscus lens with a negative power, and a diaphragm is disposed on the object side of the first lens.
(Type 2)
A lens system in which the first lens is a biconvex lens with a positive power, the second lens is a meniscus lens with a positive power, and a diaphragm is disposed on the object side of the first lens.
(Type 3)
A lens system in which the first lens is a meniscus lens with a positive power with its concave surface facing the object side, the second lens is a meniscus lens with a positive power, and a diaphragm is disposed on the object side of the first lens.
However, the lens systems belonging to those types 1–3 have the following problems.
In the lens systems belonging to Types 1–2, the object-side face of the first lens has a protruded shape towards the object side. Thus, it is not possible to achieve both the high telecentricity and back focus distance at the same time.
In the lens system belonging to Type 3, the second lens has positive power so that the chromatic aberration cannot be well-corrected even if dispersions of the first lens and the second lens are made different from each other. Particularly, an auto-focus mechanism has recently been mounted on a small-size camera that is used for a portable telephone and the like with a built-in camera. Accordingly, in the lens system used for the camera with such auto-focus mechanism, it is extremely important to well-correct various aberrations on the optical axis (particularly, the axial chromatic aberration) in order to precisely detect the best focus position near the center of the screen. In the lens system belonging to Type 3, it is difficult to well-correct such axial chromatic aberration, and the performance of the auto-focus mechanism cannot be fully utilized.
Therefore, it is the fact that there has not been proposed an effective lens system which can further improve the optical performance while being reduced in size and weight.
The present invention has been designed in view of the aforementioned problems. The object of the present invention therefore is to provide an imaging lens system which, while reduced in size and weight, can fully meet the demand for further improvement in the optical performance and also improvement in the productivity.